This disclosure relates generally to the field of magnetoresistive random access memory (MRAM), and more specifically to spin momentum transfer (SMT) MRAM.
MRAM is a type of solid state memory that uses tunneling magnetoresistance (MR) to store information. MRAM is made up of an electrically connected array of magnetoresistive memory elements, referred to as magnetic tunnel junctions (MTJs). Each MTJ includes a magnetic free layer having a magnetization direction that is variable, and a magnetic fixed layer having a magnetization direction that is invariable. The free layer and fixed layer are separated by an insulating non-magnetic tunnel barrier. An MTJ stores information by switching the magnetization state of the free layer. When the magnetization direction of the free layer is parallel to the magnetization direction of the fixed layer, the MTJ is in a low resistance state. When the magnetization direction of the free layer is anti-parallel to the magnetization direction of the fixed layer, the MTJ is in a high resistance state. The difference in resistance of the MTJ may be used to indicate a logical ‘1’ or ‘0’, thereby storing a bit of information. The MR of an MTJ determines the difference in resistance between the high and low resistance states. A relatively high difference between the high and low resistance states facilitates read operations in the MRAM.
The magnetization state of the free layer may be changed by a spin torque switched (STT) write method, in which a write current is applied in a direction perpendicular to the film plane of the magnetic films forming the MTJ. The write current has a tunneling magnetoresistive effect, so as to change (or reverse) the magnetization state of the free layer of the MTJ. In STT magnetization reversal, the write current required for the magnetization reversal is determined by the current density. As the area of the surface in an MTJ on which the write current flows becomes smaller, the write current required for reversing the magnetization of the free layer of the MTJ becomes smaller. Therefore, if writing is performed with fixed current density, the necessary write current becomes smaller as the MTJ size becomes smaller.
MTJs that include material layers that exhibit perpendicular anisotropy (PMA) may be switched with a relatively low current density as compared to MTJs having in-plane magnetic anisotropy, also lowering the total necessary write current. However, MTJs made using PMA materials may have a relatively low MR because of structural and chemical incompatibility between the various material layers that comprise a PMA MTJ. A relatively low MR may result in difficulty with read operations in the STT MRAM, as the difference in resistance between the high and low resistance states of the MTJs will also be relatively low. In a PMA MTJ, the fixed layer and the free layer may be magnetized in directions that are parallel or anti-parallel to one another, and the fixed layer may apply a relatively strong dipolar field to the free layer. The fixed layer dipolar field may offset the free layer loop by about 1000 oersteds (Oe) or more. The free layer Hc needs to be higher than the offset field from the fixed layer; otherwise, there is only one stable resistance state instead of the two stable resistance states (referred to as bistability) needed to store information in the MTJ.
MRAM is formed from a layered sheet comprising a magnetic stack of the various MTJ layers that is patterned to form individual MTJs. The MTJs may take the form of relatively small cylinders, each comprising the layered magnetic stack. In a sheet film, there is a Neel coupling between the various layers, and the fixed layer dipolar field is not present. The fixed layer dipolar field becomes apparent after the MTJs are patterned, as the dipolar field originates at the edge of the MTJ device. The fixed layer dipolar field becomes stronger as the MTJs are made smaller and is non-uniform across an MTJ. The fixed layer dipolar field creates a number of issues in MTJ devices, including increasing the free layer loop offset and the minimum required free layer Hc needed to ensure bistability of the MTJ. The minimum free layer Hc must be maintained across the full temperature operating range of the device. The fixed layer dipolar field may also change the switching mode of an MTJ, and the impact on device functionality increases as MTJ size is scaled down.